I replaced the stock mufflers with 2 supertrapp 2" to 4". I am going to rejet it so it will have the right mixture but can it do anything else harmful? I was considering using one of the pipes I have and doing a 4 to 1 but thats alot of work if im just going to loose a little bottom end.

I just went through an exhaust change a couple of months ago. As my mechanic explained it to me even straight pipes is enough back pressure for the bike to run but it will not run correctly and can burn up valves rather quickly. Once it is rejetted and carbs reset it should not be damaging to the engine but the increase in horsepower is so negligable as not to be worth the expense of changing the exhaust configuration. I did it because when I bought it the pipes were rust rotted and 4 into 1 was a much cheaper replacement than a 4 into 2. Of course I may be wrong and if any other members give you different advice you should listen to them.

dcesa, Just changing the muffler alone (assuming you still have the factory airbox) to a SuperTrapp won't hurt that much. You will loose a bit of mid-range performance if you run the muffler open with no baffles. Using the SuperTrapp on a 4-1, you will loose noticeable mid-range throttle response, and will have to rejet, and retune the mixtures.

I am guessing by what you say the majority of the back pressure is developed in the collector box? Should I rejet mine after the 4into 1 change? I have not done a color tune but my bike seems to perform great and no different than when I bought it.

Of course that was as of last week before jacka$$ wrecked it and smashed my new exhaust!

Right, the majority is. If you change to a 4-1, the system open up significantly, and re jetting comes into play. If you do the color tune, you are likely to find it running a bit lean. Check you plugs... if they are whitish in color, and not grocery store bag brown/tan, then you are running lean.

As for the JA smashing the exhaust, the smashing has constricted it and restored a bit of the back pressure... (Not trying to be an Ahole, but humor is good for the sole )

Yes humor is great and I appreciate the laugh. unfortunately it is smashed almost flat and ripped open about 6in past where the downpipes join together. Because of all the other damage I'm starting the rebuild this weekend as soon as parts arrive. At least the kid is paying for all my parts now. I should make him do the labor too but I want to do it myself for the experience. I guess now is a good time to check valves, clean carbs, sync and colortune. I wish chako took credit cards directly I would order the parts from him but I need this stuff here this week if I want to keep my job.

So, as the OEM system is relatively irreplaceable and fairly worn on most bikes, ignoring the cosmetic wear, how can we find out what the correct back pressure numbers *should* be and what the back pressure numbers *are* of the current systems available? Is it possible to find one with the same back pressure so we don't need to rejet or can we modify an after market exhaust to provide the correct amount of back pressure so we don't need to rejet? These are the questions for which I'd like the answers ...

I am a chemist not an engineer but as I understand it the reason you burn exhaust valves up if you reduce backpressure is it allows for more air to be pushed through the system. Stoichiometry tells us the optimal ratio of oxygen to gasoline is around 15:1 by volume and can be calculated as a chemical equation. Lean mixtures ie. less than the optimal ratio burn much hotter (think forced air fires to achieve hotter temps for forging metals ie blacksmith bellows). I would think it would be possible to just pack some heat resistant material into the end of the exhaust to restrict airflow but there are a lot of other factors in an exhaust like pressure waves that might make my conclusion a false one. OK you XJ Wizards I'm ready for my spanking on this. Fire away.

I was wondering that same thing, about just packing some heat resistant material... Would love to hear why it isn't that simple.

Here is my idea for an experiment. Take an intact stock exhaust from an XJ750 and Hook up the downpipes to a compressor with a pressure guage ( a home made plenum and duct tape may be involved) and write down the pressure. Then take my MAC 4 into 1 and do the same thing. Restrict the output (perhaps with washers of variuos sizes at the opening or packing in fiberglass) untill the pressure readings match. Then run the bike with each exhaust using the colortune system and see if it changes the nature of the combustion chamber flame or affects the way the engine idles or changes the temp of the engine block using an infrared guage. Anyone want to have some science/mythbusters style fun next week?

The MAIN reasons for needing Back-pressure on AIR-COOLED Engines is to Prevent Overheating damage to Piston Crowns, Combustion Chambers, Valvde Faces and Seats.

When there is NO Backpressure, the inertia of Exhaust to the atmosphere creates a flow which evacuates the Cylinder of left-over gasses which MIX with the next INTAKE Charge.

With these gasses evacuated and the amount of Fuel "FIXED" ... The Cylinder fills with MORE Air. The A/F Ratio has no Sensors or means to correct itself.
Consequently, ... there becomes and abundance of AIR causing the MMixture to Ignite and BURN HOTTER and FASTER.

There is NO Water Jacket to control HEAT.
The Engine cannot handle the excess heat and lean burn.
Oil sheen gets consumed as Fuel.
Temperatures rise.

Heat cannot get dispelled fast enough for some parts and they begin to wear out of fail prematurely.

That sounds like a fancy way of saying what I said earlier. Allows for more air through the system causing a hotter burn. The question I have is can you increase the backpressure in an aftermarket exhaust system by just packing in something to restrict airflow or are there problems related to harmonic pressure wave timing at the exhaust valves. I don't want to have to rejet with my 4 into 1

I figure you are the man to have the answer that question. Weren't you the one that had that POD modification thread I read? It sounds like you are the master engineer of this site. I'm not being flippant I really think you have mad engineering skills.

Look at the "Tunable Baffle" illustration on this site.

Tunable: www.nightrider.com/bik...xhaust.htm

Prefabricated: www.jcwhitney.com/moto...j1s17.jcwx

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